The invention relates to a structural strut, and, more particularly, to a structural load bearing beam or strut having a U-shaped surface including upstanding side walls comprised of opposed tapers for selectively shifting the neutral axis of the beam and facilitating maximum structural efficiency. It further relates to trusses and similar structures formed of such struts or beams.
Heretofore, structural assemblies such as girders, trusses and lattice beams have been constructed from structural struts of conventional design by welding, riveting and the like. Strut and chord elements have been used which have conventional cross sectional shapes in most prior art constructions. These prior art configurations comprise, in the main, L-shaped members, U-shaped channels, and I-beams, each having generally uniform flange and web configurations along the length thereof. The connection points of the terminal ends of these members with transverse chord members of a truss are, however, very limited. Generally, there is not sufficient cross-sectional area for member connection without the use of "gusset plates" of the type now used in conventional trusses. The yield strength of such component members and assemblies is also known to be limited by the material and shapes of the conventional component flanges in the respective planes of bending thereacross.
Conventional structural members illustrating prior art features of structural strut configurations, as well as various truss and girder assemblies, are set forth and shown in the following patents:
______________________________________ 573,151 P. Johnson Dec. 15, 1896 2,156,818 F. N. Ropp May 2, 1939 2,308,565 H. L. Mitchell Jan 19, 1943 2,405,917 M. Watter Aug. 13, 1946 3,334,461 F. L. York Aug. 8, 1967 3,353,320 A. R. Grasis Nov. 21, 1967 3,656,270 Boris Phillips Apr. 18, 1972 4,062,167 Tyrell T. Gilb Dec. 13, 1977 ______________________________________
Structural struts are used as braces in the fabrication of buildings, bridges, trusses, and the like. One of the most common brace or truss configurations is comprised of horizontally disposed top and bottom chords having a plurality of strut sections angularly secured there between. In one conventional embodiment of a building construction, the top chord supports a roof structure with the bottom chord supporting a ceiling. Such truss configurations may, however, also be utilized as bridges, ramps, and shelves in both horizontal and vertical configurations for affording the same structural integrity in the associated structure. Trusses are also employed as tray supporting structures inside vapor-liquid contact towers of the kind employed in the chemical and petroleum processing industries.
In conventional horizontal truss configurations, the combined stress through a section of the upper chord is the sum of compression stresses caused by truss action and a bending stress attributable to loading between support points. The total or combined stress to a section of a lower chord is the sum of tension stress caused by truss action and a bending stress caused by the load between the support points. It may thus be seen that the top chord is in compression while the bottom chord is in tension in most horizontal configurations. The angularly disposed strut members, therebetween then distribute these loads and provide structural integrity therethrough. The angular position of the particular strut members as well as the cross sectional configuration thereof establishes the loading parameters and capabilities of the associated assembly.
The strut or brace members utilized between elongated chord elements of a truss assembly are conventionally fabricated along well defined standards. The technology of such strut designs includes a portion of the study of the mechanics of solids. Within this field of study, various parameters are defined and go into the analysis and design of strut and truss configurations. One such parameter is "neutral axis" which is simply a zone of zero stress or strain as well as being the centroid or center of gravity of an elongated member which is subject to bending loads. In a symmetrical member, such as a U-shaped channel of conventional design, the neutral axis lies along the center of the channel. In an L-shaped angle member, the neutral axis lies toward the orthogonal or L-shaped side wall thereof. The neutral axis is in essence shifted by the presence of the upstanding wall, or flange. In a conventional truss construction, it is considered good design practice to arrange chords and struts so that at joints the neutral axes of the several members forming the joint intersect at a point, or form the smallest practical triangle as approximation of a point intersection. Conventional struts in truss constructions achieve or approach this result by providing a sufficiently wide strut and chord sections as well as connection plates for attachment to the sections.
Conventional truss designs incorporate load bearing members with large attachment areas for securement to intersecting chord members and/or adjacent struts. Very often "gusset" plates are utilized for providing the requisite surface area and weld regions for structurally sound interconnection. The utilization of gusset plates is an additional expense in material and labor and an added weight factor. The position of the neutral axis within the individual struts is likewise a consideration in the over all design of the structure. Conventional channels and I-beam members having centrally aligned neutral axes do not lend themselves to angulated interconnection, in a close spacing where the lateral flanges interfere with one another. The most predominant problem is the parallel relationship between the neutral axis of the strut and the upstanding flange of the U-channel or I-beam which necessitates increased chord width and/or gusset plates for structural interconnection across the intersection of the neutral axes. Additionally, strut loading requires a specifically definable bending strength which may not be uniform along the whole length of the strut due to variations of loading cross the truss. For this reason, struts of uniform cross section often present added weight to a lattice beam structure by supplying unnecessary strength and material in areas of relatively low loading. One problem aggravates the other in such designs because excess weight necessitates increased beam strength.
It would be an advantage, therefore, to overcome the problems of the prior art by providing a strut having the requisite bending strength with a minimum of material and in a configuration affording maximum strength through interconnection in a minimum of space. Such a method and apparatus is provided in the present invention wherein a strut and method of manufacture is disclosed having at least one surface of U-shaped cross section and flanges formed with opposing tapers. The maximum height of each flange is determined by the maximum bending strength necessary for the particular loading configuration in that area of the strut. By reducing the height of the upstanding side wall section adjacent that portion of the strut, the neutral axis extending therethrough is shifted toward the side of the strut having the proportionally greater flange region. This shifting relationship propogates along the strut as the wider wall section tapers downwardly and the smaller wall section tapers upwardly. This opposing flange taper functionally shifts the neutral axis along a straight line extending between said walls whereby the strut can be secured at opposite ends in a minimum of space and with precise intersection of neutral axes. Material and weight is saved by forming each end of the strut of the present invention with a flat web surface area in a shape facilitating mating abutment and welding one to another and to an associated chord member.